Aromatic isocyanates are used as starting materials for the preparation of polyurethane and, commercially, they are usually prepared by reacting aromatic amines obtained by hydrogen-reduction of aromatic nitro compounds with phosgene. This method, however, is rather complicated and, furthermore, has disadvantages in that it utilizes phosgene which is a toxic gas, and hydrogen chloride is generated at the phosgenation reaction, causing the problem of corrosion of equipment.
In recent years, therefore, several methods of production of aromatic isocyanates not using phosgene have been proposed. For example, there is known a method in which aromatic nitro compounds are reacted with compounds containing a hydroxy group and carbon monoxide in a liquid phase in the presence of a catalyst to prepare the corresponding aromatic carbamates and, thereafter, the carbamates are thermally decomposed to obtain aromatic isocyanates. This method requires a catalyst exhibiting high catalytic activity in a small amount in the preparation of aromatic carbamates. Conventional catalysts which have heretofore been proposed include a catalyst comprising (1) a platinum group metal, e.g., palladium, (2) a Lewis acid, e.g., ferrous chloride, and (3) a tertiary amine, e.g., pyridine (see Japanese Patent Application (OPI) No. 98240/76(the term "OPI" is used herein to mean a "published unexamined Japanese patent application")), and a catalyst comprising (1) palladium, (2) vanadium oxychloride, and (3) a tertiary amine (see Japanese Patent Application (OPI) No. 22339/79). Although these catalysts have fairly high catalytic activity, there has been proposed a further improved catalyst comprising (1) a platinum group metal, (2) a Lewis acid, e.g., ferrous chloride, and (3) an oxide or an acid salt of a metal, e.g., vanadium (see Japanese Patent Application (OPI) No. 128550/79).
These conventional catalysts, however, are not considered to have sufficient catalytic activity for practical use and, furthermore, since considerably large amounts of iron and/or vanadium compounds are used, they suffer from disadvantages in that the amount of the metal or metals being incorporated into the desired product crystals is increased when the crystals are separated by crystallization after the completion of the reaction.
As a result of extensive investigations to overcome the above-described problems and to provide a catalyst which exhibits a high catalytic activity even in a small amount, it has been found that a catalyst comprising a platinum group metal, vanadium, iron, a halogen atom, and a tertiary amine wherein the amounts of vanadium and iron used are reduced and the amount of the halogen atom to the total atom of the metals is controlled within a specific range prevents corrosion of equipment, inhibits the formation of by-products, particularly those which cannot be converted into the desired product, and produces the desired aromatic carbamate in high yield.